IK Multimedia is making it easier to bring a mini music studio on the road, especially if you have a newer iPhone. It just introduced an iRig Micro Amp that produces 15W while touting a USB interface, giving you a reasonably powerful yet portable amplifier with tone processing on iOS devices, Macs and PCs of all stripes. You don't need a headphone jack here, folks. It's larger than the company's earlier Nano Amp, but still compact enough that you can toss it in your backpack for impromptu practice sessions.
The amp runs either on AA batteries or a 9V power supply, and includes both auxiliary input and a headphone output jack. It'll be much more expensive than the Nano Amp when it goes on sale in March for $150, but it might be the better deal if you want a little oomph when you don't want to be tied down by larger conventional amps.